Process and apparatus for purifying and separating compressed gas mixtures



Dec. 4, 1962 c. MATSCH ETAL 3,066,493

PROCESS AND APPARATUS FOR PURIFYING AND SEPARATING COMPRESSED GASMIXTURES Original Filed Aug. 12. 195'? 2 Sheets-Sheet 1 .F W H mm EmOmmwwEvzOU l INVENTORS WILBUR H. LAUER EDWARD F.YENDA.LL LADISLAS C.MATSCH HELMUT KOEHN wamflw ATTORNE IXTURES 2 Sheets-Sheet 2 ETAL RPURIF'YING AND PARATUS FO COMPRESSED GAS M L. C. MATSCH PROCESS AND APSEPARATING Original Filed Aug. 12. 1957 Dec. 4, 1962 8V W W ATTORNEYUnited States Patent Ofifice EfifibAhS Patented Dec. 4, 1952 3 $66,493PROCESS AND AhPjtPtATUS FOR PUREFYING AND SEPARATING COMPRESSED GAS MIX-TUBES Ladislas C. Matsch and Edward F. Yendall, Kenmore,

Wilbur il'. Latter, Snyder, and l lelmut Koehn, Tonawanda, N.Y.,assignors to Union Carbide Corporation, a corporation of New YorkOriginal application Aug. 12, 1957, Ser. No. 677,606,

new Patent No. 2,984,079, dated May 16, 1961. Divided and thisapplication Oct. 11, 1960, Ser. No. 75,647

4 Claims. (Cl. 62-14) This invention relates to an improved process ofand apparatus for purifying and separating compressed gas mixtures, andmore particularly to improved process and apparatus for the separationof water and carbon dioxide impurities from compressed air prior to lowtemperature rectification of such air into air components.

This application is a division of our application Serial No. 677,606,filed August 12, 1957, now Patent No. 2,984,079, dated May 16, 1961.

Atmospheric air contains substantial quantities of water and carbondioxide impurities, and unless these impurities are removed by chemicaltreatment of the air, or by adsorption therefrom, they will deposit assolid particles on the air side heat exchange surfaces as the air iscooled. This causes considerable difiiculty, because if such depositionis continued the air side heat exchange surfaces become coated withthick layers of solid particles thus reducing heat transfer efliciency.Eventually these surfaces will plug up completely, making the airseparation process inoperative. One solution to this problem is toutilize duplicate heat exchangers piped in parallel so that a cloggedheat exchanger may be thawed while the other is in use. However, suchduplication is an expensive solution because the heat exchangersrepresent a major item of air separation plant investment cost.

In air separation plants employing relatively low air supply pressures,most of the Water and carbon dioxide impurities are removed from theincoming air and deposited in a reversible heat exchange zone by heatexchange with outgoing air separation products. This zone may compriseheat exchangers of the regenerative or passage exchanging types. Inorder to avoid a build-up of ice and carbon dioxide solid particles insuch heat exchange zone, the zone must be self-cleaning. This means thatall of the impurities deposited in the zone during an air intake strokemust be evaporated and swept out during the next succeeding product gasstroke. The self cleaning condition may not be achieved by simply assingall of the outgoing product gas through the reversible heat exchangezone because compressed air, especially at low temperatures, has asubstantially greater specific heat than the non-compressed airseparation products, e.g. oxygen and nitrogen.

The prior art has devised many ways of alleviating this condition, oneof which involves partially cooling the incoming air stream in thereversible heat exchange zone and depositing the water impurity in suchzone. A minor portion of the partially cooled air stream is withdrawnfrom the zone and separately cleaned, while the major portion of the airstream is further cooled in the reversible heat exchange zone. Most ofthe major portions carbon dioxide content is removed by deposition insuch zone. Since by this arrangement, the volume of outgoing airseparation products passing through the colder part of the zone issubstantially greater than the volume of incoming air passing throughthis part, the reversible heat exchange zone can be made selflcleaning.One method of cleaning the diverted minor portion of partially cooledair, or side-bleed, is to chill the latter well below the carbon dioxidedeposition point by direct mixing with a small part of the furthercooled or cold end air having passed through the entire length of thereversible heat exchange zone. Unfortunately this scheme has severalimportant disadvantages. it direct mixing is used, local precipitationof carbon dioxide is likely to occur at the point of mixing, thusrequiring a filter to remove the solid particles. Also, if colder air ismixed with the side-bleed air, a larger quantity of air must beprocessed by the filter, and a larger filter must be used to avoid ahigher pressure drop. For control purposes, it is desirable to maintainthe pressure drop in the side-bleed circuit as low as.possible.Furthermore, to mix cold end air with the sidebleed air, the latter mustbe slightly throttled. This is because the undiverted air is subjectedto additional pressure drop in passing through the colder part of thereversible heat exchange zone. As a result of the side-bleed throttlingnecessity, if any part of the carbon dioxide-free throttled side bleedair is subsequently to be bypassed to the cold end air stream, thelatter must also be slightly throttled to obtain flow in the desireddirection. Throttling of the cold end air is undesirable as itsubstantially increases the air compression power costs.

Another problem connected with the side-bleed method of unbalancing areversible heat exchange zone for selfcleaning is determining thetemperture level for such side bleeding. The ideal temperature range forsidebleeding regenerators or reversing heat exchangers is C. to C. Thereare several reasons for this selection, as follows:

(1) The side-bleed air volume necessary for selfcleaning is less atWarmer levels than at colder levels. For example, the required volume atthe C. level is 30% to 40% greater than at the -l00 C. level.

(2) A warmer side-bleed point avoids approaching too closely the levelat which carbon dioxide begins to deposit, e.g. approximately l34 C. for75 psi air. The safe margin for trouble-free operation of regeneratorsor reversing heat exchangers is substantially reduced by lowering theside-bleed level below 120 C.

(3) In the case of reversing heat exchangers, the units are normallyavailable in standard lengths, and this conveniently fixes thetemperature between the warm and cold units at approximately -100 C.Withdrawing the side-bleed at a colder level would require remanifoldingthe complex passages at an intermediate point in the cold unit. Althoughthis can be done, it adds appreci ably to the cost of the exchangers.

One practical method of removing the carbon dioxide impurity from theair side-bleed is by gas phase adsorption using an adsorbent such assilica jel. In this manner the dissolved carbon dioxide may be removedwhile still in solution, thereby avoiding the difiiculties of solidprecipitation. Unfortunately, the ideal temperature for side-bleedingthe reversible heat exchange zone does not correspond with the idealoperating temperature range for a silica gel trap, the latter being 120C. to -13(l C. This range is advantageous because the carbon dioxideadsorbing capacity of silica gel is considerably higher at lowertemperatures.

A further problem arises if an air stream is to be expanded with theproduction of external Work and lowtemperature refrigeration. Theside-bleed air is a convenient source of warm gas for such workexpansion, but either or both its temperature and .volume may beunsuitable for such work expansion. For example, the optimum expansionturbine inlet temperature is approximately -l53 C., which of course issubstantially below either the optimum side-bleed or silica geladsorption temperature. The turbine inlet temperature is selected so asto operate as cold as possible and yet avoid appreciable aeeeaes liquidcondensation in the turbine discharge. The latter is undesirable as itproduces erosion of the turbine blades and loss of efiiciency. Also, theaddition of colder air to the side-bleed air to obtain this optimumtemperature level for the turbine Will generally result in a largervolume of air than is desired for work expansion. The quantity of air tobe work expanded is determined by a heat balance on the air separationcycle. It is undesirable to work expand more air than is required by theheat balance, because such excess air expansion would reduce theefliciency of the process and may require an oversized turbine.

Principal objects of the present invention are to provide a process andapparatus for purifying and separating compressed air utilizing aside-bleed for unbalancing the reversible heat exchange zone, means forremoving the carbon dioxide impurity from the side-bleed, and means forwork expanding at least part of the side-bleed, the steps and apparatusbeing arranged and related so that each step is conducted under ideal oroptimum conditions, the overall result being a highly eflicient and lowcost systern for separation of air into products or components.

These and other objects and advantages of this invention will beapparent from the following description and accompanying drawings inwhich:

FIG. 1 shows a flow diagram of a system for purifying an air stream andpreparing a portion of such stream for work expansion, according to thepresent invention;

FIG. 2 is a flow diagram of a modified system according to the presentinvention.

According to the present invention, an air stream at an inlet pressurebelow 150 p.s.i. is passed to a reversible heat exchange zone, partiallycooled to at least 80 C. for removal of the water impurity by depositionin such zone, and divided into major and minor portions. The minor orside-bleed portion is Withdrawn from the zone and further cooled by heatexchange with a first colder fluid to a temperature slightly above thedeposition point of carbon dioxide at the inlet pressure. The dissolvedcarbon dioxide impurity is then removed by adsorption from this furthercooled side-bleed stream. Also, the major portion of the air stream isfurther cooled in the reversible heat exchange zone, and at least mostof the carbon dioxide impurity of such portion is removed by depositionin the colder part of such zone. At least part of the further cooledcarbon dioxide-free major portion, or cold end air, is passed to arectification zone for separation into air components. The water andcarbon dioxide impurities deposited in the reversible heat exchange zoneare removed by passing at least part of the separated air componentsthrough such zone to evaporate such impurities therein for dischargefrom the zone.

In one embodiment of the invention, a minor part of the cold end air isdiverted and passed in cocurrent heat exchange with the partially cooledside-bleed air as the first colder fluid.

If an air stream is to be work expanded, this invention provides amethod of forming an expander inlet stream having suitable flow andtemperature so as to achieve optimum work expansion conditions as Wellas self-cleaning conditions. This may be accomplished by diverting aregulated part of the partially cooled carbon dioxide-free side-bleedair, slightly throttling the undiverted side-bleed, and diverting aregulated part of the cold end air to the slightly throttled undivertedside-bleed, thus providing the desired expander inlet stream.

In still another embodiment of the invention, the methods of withdrawinga side-bleed, removing the carbon dioxide impurity from such side-bleed,and Work expanding a stream including part of the cleaned side-bleed maybe combined so that each step is conducted under optimum conditions.This may be achieved by further cooling the side-bleed by heat exchangewith a first colder fluid, removing the carbon dioxide impurity from thefurther cooled side-bleed, still further cooling the cleaned sidebleedby heat exchange with a second colder fluid, and passing the stillfurther cooled clean side-bleed in heat exchange with the uncleaned orraw side-bleed as said first colder fluid. Next, a regulated part of thepartially rewarmed side-bleed or first colder fluid is diverted to thecold end air stream, the undiverted partially warmed sidebleed isslightly throttled, and a regulated part of the cold end air is divertedto the slightly throttled side-bleed. The latter stream is thenpreheated prior to work expansion by heat exchange with the carbondioxide-free further cooled side-bleed stream. In the latter step, theslightly throttled side-bleed serves as said second colder fluid.

It can be seen from the previous brief descriptions that this inventionpermits ideal operating conditions in the various steps of the airseparation cycle, the overall result being a highly efficient andeconomical method of obtaining air separation products such as oxygenand nitrogen. Furthermore, this invention eliminates the previouslydescribed disadvantages of the direct mixing and chilling method oftreating the air side-bleed. In the present invention, there is nodirect mixing with cold end air prior to carbon dioxide removal, hencethe side-bleed is cooled to the ideal temperature for carbon dioxideremoval without the necessity of using a filter or a larger silica geltrap. Furthermore, it will be noted that no throttling of the side-bleedoccurs prior to carbon dioxide removal, hence cleaned side-bleed air maybe diverted to the cold end air Without having to throttle the latterstream to induce flow in the desired direction.

Referring now to the drawings and particularly to FIG. 1, air iscompressed in compressor 10 to a pressure of less than 150 p.s.i.g. andpreferably about p.s.i.g., and the heat of compression may be removedby, for example, a water-cooled exchanger (not shown). The compressordischarge air stream passes through conduit 11 and reversing valves 12to a reversible heat exchange zone 13, which may comprise regeneratorsor reversing heat exchangers. In zone 13 the air stream is cooled byheat exchange with air separation products such as oxygen or nitrogen,such products entering the cold end of the reversing heat exchange zone13 through conduit 14 and reversing valves 14a therein, and emergingthrough conduit 15 at the warm end of zone 13. The manner of cooling theair stream by heat exchange either with a colder fluid in an adjacentpassageway, or through an intermediate refrigeration storage means suchas regenerative packing, is well-known to those skilled in the art anddescribed in Frankl U.S.P. 1,890,646 for regenerators, and TrumplerU.S.P. 2,460,859 for reversing heat exchangers. Reversing valves 12 and1411 are suitably connected to each other and to the zone 13 in order toachieve this cyclic heat exchange.

The water content of the inlet air stream is removed by deposition inthe warmer part of the reversible heat exchange zone 13, and such streamis divided into major and minor portions by withdrawing a minor portionor side-bleed at approximately -100 C. through conduit 16 and valve 17therein. The side-bleed air may constitute approximately 3% to 15% ofthe total inlet air stream, and preferably about 10%. One purpose of theside-bleed is to bypass a sufflcient portion of the inlet air streamaround the colder part of the reversing heat exchange zone 13 so thatthe flow ratio of outgoing air separation products to inlet air will besufliciently increased to achieve a self-cleaning temperature pattern inthe zone 13. As previously discussed, it may be preferably to withdrawthe side-bleed air at the C. to C. level instead of a lower temperaturelevel to minimize the side-bleed flow and positively avoid carbondioxide deposition in the heat exchange zone 13 above the side-bleedlevel.

The partially cooled minor air portion or side-bleed is conductedthrough conduit 16 to passageway 18 where it engages in cocurrent heatexchange (i.e. in the same direction) with a first colder fluid inpassageway 19, and

is itself further cooled to approximately l C. This is an optimumtemperature for silica gel adsorption of carbon dioxide from 75 psi. airwhich still provides a safe margin above the deposition point of carbondioxide at this pressure. At these conditions, carbon dioxide beginsdepositing at approximately -134 C., and it the side-bleed air werecooled appreciably below about -l20 C., it is probable that some carbondioxide deposition would occur in the heat exchange zone 13 above theside-bleed level, in passageway 18, and in the interconnecting piping,and this could eventually necessitate a shut-down for thaw. The furthercooled side-bleed is passed from passageway 18 through conduit 19:: andinlet valves 20 into one or the other of a pair of the silica gel traps21 for removal of the still dissolved carbon dioxide by silica geladsorption. These gel traps are provided in duplicate and piped inparallel for alternate operation so that when one gel trap becomesloaded with carbon dioxide, the side-bleed air may be diverted to theother gel trap having previously been purged and reactivated by meansnot illustrated. As previously discussed, it is preferable to conductthe adsorption step at relatively low temperatures, such as l20 0,because the carbon dioxide adsorptive capacity of silica gel is higherat lower temperatures. The carbon dioxide-free side-bleed air emergesthrough silica gel trap discharge valves 22 into conduit 23.

Meanwhile, the major portion of the inlet air stream is further cooledin the reversible heat exchange zone 13, and most of its carbon dioxideimpurity is removed by deposition in the colder part of such zone. Thefurther cooled major portion is discharged from the cold end of zone 13at approximately 173 C. into conduit 24, and thence passes to cold endtrap 25 for silica gel filtration and adsorption of any residual carbondioxide not previously removed. The carbon dioxide-free further cooledair, or cold end air, is discharged from silica gel trap 25 into conduit26.

In this particular cycle, it is desirable to expand an air stream withthe production of external work, but the carbon dioxide-free side-bleedair stream is two warm (-120 C.) and may be too large or too small involume for optimum work expansion conditions. An air stream havingsuitable flow and temperature for work expansion may be formed by firstdiverting a regulated part e.g. of the side-bleed through conduit 27 andregulating valve 28, and still further cooling such diverted part toapproximately l C. by passage through passageway 23 in heat exchangewith a colder fluid flowing through passageway 39. Such colder fluid maybe, for example,

nitrogen product from the rectification column. This diverted streamstill further cooled in conduit 27 is then mixed with the cold end aireither downstream or upstream of the cold end silica gel trap 25. Thefurther cooled major portion or cold end air is passed through conduit2a to the rectification column (not shown) for separation into productssuch as oxygen and nitrogen in a manner Well-known to those skilled inthe art. Alternatively, the diverted part in conduit 27 may be passeddirectly to the rectification column instead of united with cold endair.

In the previously described system, it is to be noted that air flow isobtained in the desired direction, that is, from the side-bleed streamto the cold end stream, because the pressure drop in the side-bleedcircuit is less than the pressure drop through the colder part of thereversing heat exchange zone 13 and the cold end gel trap 25. Since coldend air is to be diverted from conduit 26 to the carbon dioxide-freeside-bleed in conduit 23 to cool the latter stream, it is necessary toslightly throttle the side-bleed by means of valve 31 to a pressure justbelow the pressure of the cold end air conduit as. A regulated part,e.g. 18% of the latter, is diverted through conduit 32 and passedthrough passageway 19 in cocurrent heat exchange with the side-bleed inconduit 18 to further cool 6 the latter as said first colder fluid. Abypass conduit 33 and valve 34 are provided to insure flexibility ofoperation.

Cocurrent heat exchange assures that the metal wall temperature in theheat exchanger formed by passageways 18 and 19 will not drop below -134C. at any point, and hence carbon dioxide will not be deposited therein.Cocurrent flow, although less eflicient than countercurrent flow, ispreferred when cold end air is used for cooling the side-bleed air,since this arrangement places the warmest portion of the side-bleed air(at C.) adjacent to the coldest portion of the cold end air (at -l73C.). Thus, the arithmetic average of the temperatures at the inlet endof the cocurrent heat exchanging passageways 18 and 19 is about l36 C.,and the metal wall temperature can easily be raised well above thecarbon dioxide deposition range by increasing the heat transfer surfaceon the air side-bleed passageway 18, which is the warm side of the heatexchanger. In practice, it is preferable to provide a warm side hA value(heat transfer coeflicient area) about 25% greater than the hA value forthe cold side.

It is possible to employ countercurrent heat exchange, but in this casethe coldest portion of the side-bleed air (at l20 C.) is adjacent to thecoldest portion of the cold end air at -173 C., and the arithmeticaverage is l45 C. Thus, it is more difficult to design and operate acountercurrent heat exchanger to maintain the walls of passageway 18above the critical l34 C. temperature.

The partially rewarmed diverted cold end air in conduit 3'2. mixes withthe slightly throttled side-bleed from conduit 23 to form a turbineinlet stream in conduit 35. The flow of this stream and the divertedcold end air in conduit 32 are regulated by valve 36 in conduit 35. Inthis manner, an expander inlet stream is formed so as to achieve optimumwork expansion conditions, and the regulated inlet stream in conduit 35at about -l53 C. enters turbine 37 for work expansion therein to about 3psi. and l83 C. The turbine discharge stream in conduit 38 may either bepassed to the rectification column for separation into air components,or united with the portion of these components passing to the reversibleheat exchange zone 13 for cooling and partially cleaning the inlet airtherein. A further possibility is dividing the work expanded side-bleedair so that a part thereof may be directed to each of the aforementionedpoints. in any event, this stream contains a substantial quantity ofrerigeration, and the cycle efiiciency is greatly improved when suchrefrigeration is recovered.

Referring now to the embodiment illustrated in FIG. 2, the featureswhich are similar to those shown in PEG. 1 are designated by similarreference numerals. The process and apparatus differ in certainparticulars in that the further cooled carbon dioxide-free side-bleed inconduit 123 is still further cooled to about-l35 C. in pasageway 156 byheat exchange with a second colder fluid in passageway 151, and is thendirected to passageway 13.9 where it countercurrently cools the carbondioxide laden side-bleed air stream in passageway 11? as the previouslymentioned first colder fluid. in this case, countercurrent cooling maybe used without the special precautions necessary when using cold endair at 173 C. because this coolant at C. is not cold enough to producecarbon dioxide deposition in passageway 11S.

Again in the FIG. 2 modification of the invention it is desirable towork expand an air stream, hence a suitable stream is formed to attainideal work expansion conditions. A regulated part, e.g. 50 of thepartially rewarmed side-bleed air emerging from passageway H9 atapproximately ll0 C. is diverted through conduit 1.27 and regulatingvalve 128 to the cold end air in conduit 12s, and the undivertedside-bleed is slightly throttled through valve 131. A regulated part,e.g. 17%, of the cold end air at approximately l73 C. is diverted byconduit 132 and mixed with the slightly throttled sidebleed in conduit135 with e result that this combination stream has a temperature ofapproximately l60 C. which is too cold for direct Work expansion. Thegas flow in conduit 135 is regulated by valve 136, and

been described in detail, it is contemplated that modifications of t'-eprocess and the apparatus may be made and that some features may beemployed Without others, all Within the spirit thereof as set forth inthe claims. The principles of the invention may also be applied to thepurification and separation of other low-boiling gas mixtures containingWater and carbon impurities.

What is claimed is:

1. A process for the separation of Water and carbon dioxide impuritiesfrom compressed air prior to low-temperature rectification of the airinto its components including the steps of providing an air stream at aninlet pressure below 150 p.s.i., passing such stream to a reversibleheat exchange zone, partially cooling the air stream in such zone to atleast 80 C. and removing the Water impurity by deposition in the heatexchange zone, dividing the partially cooled air stream into major andminor portions, Withdrawing the minor portion from said reversible heatexchange zone and further cooling such portion by heat exchange with afirst colder fluid to a temperature slightly above the deposition pointof carbon dioxide at said inlet pressure, removing the dissolved carbondioxide impurity from the further cooled minor portion of said air steam by adsorption, still further cooling the carbon dioxide-free minorportion by heat exchange with a second colder fluid so as to comprisesaid first colder fluid which further cools the withdrawn minor portion,removing at least most of the carbon dioxide impurity from the partiallycooled major portion of said air stream by further cooling anddeposition thereof in said reversible heat exchange Zone, passat leastpart of further cooled major portion to a rectification Zone forseparation into air components, and removing the Water and carbondioxide impurities deposited in said reversible heat exchange zone bypassing at least part of the separated air components through the zoneand evaporating such impurities therein for discharge from such zone.

2. A process according to claim 1 for the separation of water and carbondioxide impurities from compressed air prior to low-temperaturerectification, including the step of diverting a minor part of thefurther cooled major portion of said air stream and mixing such divertedpart with the warmed first colder fluid so as to comprise said secondcolder fluid Which further cools the carbon dioxide-free minor portionoi'said air stream and is itself prheated, and expanding such preheatedmixture from: approximately said inlet pressure to a low pressure with.

production of external work. 3. Apparatus for the separation of waterand carbon dioxide impurity-containing air by low-temperature rec--tifieation including a rectifying device; means by which inlet air issupplied at a pressure below p.s.i.; a re-- versiale heat exchange zonefor partially cooling the inletv air to at least 8G C. so that the waterimpurity is deposited in such zone; means for dividing the partiallycooled inlet air stream into major and minor portions; means forwithdrawing the minor portion from said reversi'ole heat exchange zone;means for further cooling the withdrawn minor portion by heat exchangewith a first colder fiuid to a temperature slightly above the depositionpoint of carbon dioxide at inlet pressure; means comprising anadsorption trap for removing the dissolved carbon dioxide impurity fromthe further cooled minor portion; means for still further cooling thecarbon dioxide-free minor portion of said air stream by heat exchangewith a second colder fluid; means for passing the still further cooledminor portion to said means for further cooling the Withdrawn minorportion as said first colder fluid; means for further cooling the majorportion of said air stream in said reversible heat exchange zone so thatat least most of the carbon dioxide impurit is deposited in such zone;means for passing at least part of the further cooled major portion tosaid rectifying device for separation into air components; and means forpassing at least part of the separated air components through thereversible heat exchange Zone to evaporate and discharge the previouslydeposited air impurities from such zone.

4. Apparatus according to claim 3 for the separation of water and carbondioxide impurity-containing air including means by which a part of saidfurther cooled major portion is diverted and mixed with the warmed firstcolder fluid to comprise a Work expander inlet stream; means forpreheating such inlet stream as said second colder fluid by heatexchange with said carbon dioxide-free minor portion of said air stream;and means comprising a work expander for expanding such inlet stream toa low pressure With the production of external work.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR THE SEPARATION OF WATER AND CARBON DIOXIDE IMPURITIESFROM COMPRESSED AIR PRIOR TO LOW-TEMPERATURE RECTIFICATION OF THE AIRINTO ITS COMPONENTS INCLUDING THE STEPS OF PROVIDING AN AIR STREAM AT ANINLET PRESSURE BELOW 150 P.S.I., PASSING SUCH STREAM TO A REVERSIBLEHEAT EXCHANGE ZONE, PARTIALLY COOLING THE AIR